I set my computer to print in "Grayscale" and I've told it to only use the black ink cartridge, black is cheaper than color and if I do need to print in color I can just temporarily change it back. If you don't change to "Black Ink Only" it can still be using ink from your color ink cartridge even if you are only printing black.
I've also changed my printer to print "Fast Draft", it still prints out great for coupons or just everyday things you need to print. If you are doing an important project or printing photos you can go back in and temporarily change it. Changing to Fast Draft saves a ton of ink, you'll be amazed how much longer your ink cartridge lasts.